Spectral linewidth broadening from pair fluctuations in a frozen Rydberg gas

Spectral linewidth broadening in Rydberg gases, a phenomenon previously attributed to the many-body effect, was observed experimentally almost a decade ago. The observed linewidth was typically 80-100 times larger than the average interaction strength predicted from a binary interaction. The interpretation of such a phenomenon is usually based on the so-called diffusion model, where the linewidth broadening mostly originates from the diffusion of excitations. We present a model calculation to show that diffusion is not the main mechanism of the linewidth broadening. We find that the rare pair fluctuation at small separation is the dominant factor contributing to this broadening. Our results give a width of about 20-30 times larger than the average interaction strength. More importantly, by turning off the diffusion process, we do not observe an order-of-magnitude change in the spectral linewidth.